fix: Use 3.6

Signed-off-by: Ce Gao <gaoce@caicloud.io>

cmd/suggestion/chocolate/v1alpha3/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3
+FROM python:3.6
 
 RUN if [ "$(uname -m)" = "ppc64le" ]; then \
         apt-get -y update && \

cmd/suggestion/hyperopt/v1alpha3/Dockerfile

@@ -1,10 +1,11 @@
-FROM python:3
+FROM python:3.6
 
 RUN if [ "$(uname -m)" = "ppc64le" ]; then \
         apt-get -y update && \
         apt-get -y install gfortran libopenblas-dev liblapack-dev && \
         pip install cython; \
     fi
+
 RUN GRPC_HEALTH_PROBE_VERSION=v0.3.0 && \
     wget -qO/bin/grpc_health_probe https://github.com/grpc-ecosystem/grpc-health-probe/releases/download/${GRPC_HEALTH_PROBE_VERSION}/grpc_health_probe-linux-amd64 && \
     chmod +x /bin/grpc_health_probe

cmd/suggestion/nasrl/v1alpha3/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3
+FROM python:3.6
 
 RUN if [ "$(uname -m)" = "ppc64le" ]; then \
         apt-get -y update && \

cmd/suggestion/skopt/v1alpha3/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3
+FROM python:3.6
 
 RUN if [ "$(uname -m)" = "ppc64le" ]; then \
         apt-get -y update && \